Diet is the food that is consumed by an animal. Marine animals are categorized into 3 groups: Herbivores (those that eat algae), Carnivores (those that eat other animals), and Omnivores (those that eat both plants and animals). In reality, nearly all marine fish are omnivores. Carnivores consume some plant material, often accidentally, as they hunt for herbivores. Herbivores often eat small animals as they consume algae. These small "accidental" foods are very important to the long term health of any marine organism. Providing a balanced diet in the marine aquarium must pay attention to this fact.

Foods: Foods available to the marine aquarist fall into several categories: Live Algae, Live Animal Foods, Frozen Foods, Dry Foods, Table Foods, and Home-Made Meals.

Live Algae: There are several species of algae that are available to the marine aquarist as a food source for fish. They are all good sources of vitamins, EPA (eicosapentaenoic acid), and trace minerals.

Caulerpa includes a number of species of algae that are suitable as food. It has been banned in California because of its invasiveness in coastal waters.

Gracilaria is a stringy red algae that is very nutritious.

Ulva is a green algae that grows in large wrinkled sheets, which can be torn and fed to herbivores.

Sargassum is a species of brown algae that is very nutritious.

Live Animal Foods include:

Brine Shrimp (Artemia sp.) - newly hatched forms are very nutritious for small fish, and often important to the acclimation of mandarins and gobies. Newly hatched brine shrimp (under 48 hours age) are very high in nutrition at about 70% protein and 18% lipid. They should not be fed continuously to fish. Adult brine shrimp have low nutritional value unless they were fed an enriched diet.

Grass/Ghost Shrimp - Palaemon sp. shrimp can be collected and fed to aquarium fish. They can be preloaded with nutrition algae such as Spirulina and then fed to fish.

Copepods range in size from less than a millimeter to about a centimeter in length. They are crustaceans that form an important link between algae and many of the ocean's smaller carnivores. They can be cultivated in refugiums or bought. Available forms include "Arcti-Pods&auot;, "Reef Plankton", and "Tigger Pods".

Amphipods are small crustaceans with a different body structure than copepods, being more shrimp-like. Gammarus is a common amphipod that is often transported with live rock or macroalgae. They feed on detritus and algae in a refugium or aquarium.

Feeder Fish are used to feed large carnivorous fish like lionfishes and rockfishes. Marine feeder fish are expensive and usually not offered as food. Freshwater feeder fish include mollies and mosquito fish. Goldfish are not suitable as marine fish food because they will lead to the development of liver problems in the predators that consume them.

Earthworms (Lumbricus) provide an excellent food source for marine fish. They should not be collected from areas exposed to pesticides and fertilizers. They can be shredded for smaller fish or fed whole to larger species. They are easily raised in plastic shoe box containers with potting soil and bran or oatmeal for food.

White worms (Enchytraeus sp.) are smaller than earthworms, reaching only about 2 cm. in length. They are nutritious but should only be fed sparingly because they are high in fat. They can be raised in plastic shoe box containers with potting soil and bran or oat meal for food.

Tubificid Worms include Tubifex sp. (red) and Lumbriculus sp. (black) worms that are very high in protein. Blackworms are usually cleaner than red worms because of black worms are usually cultured. They die quickly in saltwater so use a small clear bowl to lower them into the water and allow fish to eat them before they are swept away.

Mealworms are larvae of the beetle Tenebrio molitor and are easy to raise and feed to some predatory fish. They can be raised in plastic shoe boxes with whole oatmeal or wheat bran with a potato slice for food and moisture.

Frozen Foods include algae, worms, shrimp, and shellfish, among others. The freezing process alters the chemistry of the food somewhat, and vitamins C and E do decline over time. Vitamin supplements can counteract this. Defrosting the frozen food in water often causes the food to break up into bits often so small that fish cannot consume them. Furthermore, vitamins and minerals in the frozen food leach into the surrounding water and become unavailable to the fish. The excess uneaten organic material only adds to the chemical load in the aquarium water. Frozen foods are best allowed to thaw on a plastic surface and then placed directly into the aquarium for feeding.

Frozen Mysis shrimp is an excellent food source that is lacking only in pigment. Mysis are muscular freshwater shrimp that are high in protein and naturally occurring fatty acids. Since they are freshwater they should not be fed exclusively to fish. They should be thawed in a minimal amount of saltwater and then fed directly to fish to avoid fragmentation and the loss of nutrients from the shrimp.

Dry foods are processed versions of algae, shrimp, shellfish, and other foodstuffs. They have the advantage of a long shelf life. Dry foods are often lacking in some pigments and vitamins that live whole foods contain. Dry foods come in many forms, from flake to blocks, and include everything from algae to worms.

Flakes and Pellets are produced by a variety of companies and can contain a wide range of materials, from shrimp to algae. Different companies include different ingredients to produce different results in fish. Some have extra vitamins and minerals to enhance color. Others have higher protein to stimulate growth. Some types float for top feeders and other sink for bottom feeders. It is important to consider this when purchasing a dry food for a specific fish.

Spirulina is a blue green bacteria that is freeze dried to be fed to humans or fish. It is high in pigment and vitamins and is an excellent food for marine fish.

Porphyra and Palmaria seaweeds, known as Kelp and Nori, are excellent sources of vitamins, minerals, and lipids for fish. They can be bought from Oriental food stores and world wide web companies. Green seems to be the best for fish. Kelp and Nori are available as sheets, strips, or flakes.

Feeding blocks containing algae in a plaster-of-Paris cement allow fish to graze on algae as the plaster dissolves. The plaster adds calcium to the aquarium so it is harmless.

Freeze-Dried Foods include algae, worms, and shrimp. Their moisture content is removed very quickly under high pressure with a vacuum system and their nutrition remains high right up to feeding time. Freeze-dried foods are good at absorbing vitamin, mineral, and lipid supplements. Freeze-dried copepods maybe the best all-around food for marine fish with 50% protein, 35% lipids, 12% carbohydrate and carotenoid pigments. Freeze-dried krill (Euphausia) is also high in protein, fatty acids, carotenoid pigments, and vitamins. They should not be fed exclusively to fish to avoid a condition known as "lockjaw". Freeze-dried bloodworms (Chironomus) are actually insect larvae (midges). Because they are not marine, they are great for an occasional meal only.

Table Foods include vegetables and shellfish. Vegetables from the garden should only be used sparingly as marine fish food. Better types include romaine lettuce, peas, spinach, and broccoli. These should only be used sparingly because they are high in cellulose and can cause bloating in fish. All of the garden food types are high in nitrates and some contain oxalic acid (spinach) so they must be used with caution. Seafoods such as shellfish and fish are excellent sources of protein and fat for marine fish. Fresh shellfish, opened and exposed, are irresistible to some picky fish species.

Home-Made Meals can be prepared from seafood items bought at a grocery deli. Squid, shrimp, clams, scallops, mussels, and crab are good. Haddock and flounder are good fish. Avoid oily foods that help spoil aquarium water. Always use marine organisms for the food. Cut the pieces into bite-size portions for your fish and then roll them in aluminum foil or place in very small (drug-type) ziploc bags for future feedings.